(1) Field of the Invention
The present invention relates to organic electroluminescence elements, and particularly relates to manufacturing a top-emission organic electroluminescence element and a manufacturing method thereof.
(2) Description of the Related Art
The organic electroluminescence element (hereinafter, described as an organic EL element), which has a structure in which an organic layer including a luminescent layer is sandwiched between an anode and a cathode, is a device that takes out illumination by injecting holes through the anode and electrons through the cathode and recombining the injected holes and electrons inside the luminescent layer.
The organic EL element is characterized by having high visibility because of its self-luminousness and also having excellent impact resistance because it is a complete solid-state device. The use of it as a light-emitting element in a variety of display apparatuses has been receiving widespread attention (for example, see Patent Reference 1). The organic EL element has already been used for various purposes, including use for the main display of cell phones.
In the current stage of development and application, the development of an organic EL element using a low-molecular organic material for a luminescent layer is advanced. This is because the organic EL element has high luminance efficiency and long lifetime when using a low-molecular organic material for the luminescent layer (for example, see Patent References 2 and 3). The luminescent layer made of such a low-molecular organic material is manufactured using vacuum processing such as vacuum deposition.
However, the organic EL manufacturing method using such vacuum processing hardly allows mass-production of large scale organic EL panels such as 100-inch panels. This is because when using a deposition technique, it is difficult to maintain mask alignment accuracy for each color (for example, R, G, and B) in mask deposition in the process of forming the luminescent layer. In addition, in the case of using the deposition technique, problems such as the problem of film uniformity in the process of forming an electrode layer pose a technical impediment to the manufacturing of the large scale organic EL panels. In addition, in terms of costs in which the manufacturing method is reflected, the organic EL element is behind a liquid crystal display which rivals the organic EL element.
Whereas, a method for manufacturing an organic layer by wet processing has advantages in terms of: material use efficiency, manufacturing time, and manufacturing apparatus costs. In addition, when applying this method to manufacturing of the display, since pixels are separately colored in large area by a printing method, problems such as non-uniformity in a surface direction as seen in the case of vacuum deposition or deflection of a metal mask used for patterning the deposition layer are not observed.
On the other hand, in the case of forming multiple organic layers by the wet processing, there is a problem of difficulty in forming such multiple layers because dropping a solution onto an upper layer generally causes a lower layer to be eluted into the solution. Since the performance of the organic EL element can be improved by stacking multiple organic layers having various functions, this is an important problem. Therefore, the performance of the organic EL element manufactured by the wet processing is lower than the performance of the organic EL element manufactured by the vacuum deposition method. For practical application of the organic EL element using the wet processing, it is essential to select the device structure and the material appropriate for the wet processing and to develop the manufacturing method thereof.
Patent Reference 4, for example, discloses a device structure of a multilayer organic EL element manufactured by the wet processing and the manufacturing method thereof. Patent Reference 4 discloses forming, by an ink-jet method, a hole injection layer made of an aqueous organic substance PEDOT on an electrode on a substrate. From above, a material having hole transportability and including a cross linker is formed, along with an organic solvent, into a film, and a link is formed by optical treatment after the film formation so as to insolubilize the material. Next, as a luminescent layer, a third layer made of a luminescent organic material is formed into a film along with the organic solvent. Finally, the cathode is formed by deposition, so that an element is formed.
In addition, Patent Reference 5 discloses forming, as an inorganic hole injection layer, a metal-oxide layer having a large work function and thus having an advantage in hole injection in terms of energy level. Since these layers are insoluble to an organic solvent, the problem of elusion is not caused in the case of wet-applying an organic solvent from above.
Patent Reference 1: Japanese Unexamined Patent Application Publication No. 2004-171951.
Patent Reference 2: Japanese Unexamined Patent Application Publication No. 2007-288071.
Patent Reference 3: Japanese Unexamined Patent Application Publication No. 2002-222695.
Patent Reference 4: Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2007-527542.
Patent Reference 5: Japanese Patent: 3369615.
However, in the case of using the PEDOT for the hole injection layer, the deterioration of the device is accelerated by diffusion of sulfonic acid into the organic layer. In addition, a water-soluble conductive material represented by the PEDOT solution is acidic solution and has a problem of causing corrosion to equipment such as an ink-jet nozzle. In addition, the material is not a complete solution but is made up of diffused fine particles, thus causing a problem of clogging the ink-jet nozzle. Furthermore, its conductivity, which is too high, causes a leakage current to increase when even a portion of this film has contact with the cathode.
On the other hand, in the case of using a metal oxide for the hole injection layer, since the deposition method requiring a high temperature for film formation is generally used for forming a metal oxide layer having a larger work function, crystallization is promoted. When crystallization is promoted, the film becomes less planarized, so that pinholes are generated in an organic luminescent layer that is an upper layer. In addition, when increasing the thickness of the metal oxide film to avoid the generation of such pinholes, electric resistance of the film increases, and requires, as a result, a high drive voltage, thus causing power consumption to increase.
In addition, it is possible to give molybdenum oxide as a metal-oxide layer having sufficient film flatness and having a large work function. However, the molybdenum oxide is water-soluble and solves into an aqueous solution used for such processing as patterning the organic luminescent layer and forming a pixel separation bank, thus making it difficult to obtain an oxidized film having a desired flatness and thickness.
When the organic EL element is realized using the organic luminescent layer formed by a wet film forming method in order to achieve a larger area of the image display unit and reduce manufacturing costs, the hole injection layer has such problems as described above.